1. Field of the Invention
The present invention relates to a hydroform process for forming an original tube member, using a die having portions to be brought into contact with the outer peripheral surface of the original tube member, and also relates to a hydroform product obtained by the hydroform process.
2. Description of the Related Art
In the field of reinforcing members for reinforcing each section of vehicles such as automobiles, the employment of reinforcing members formed by hydroforming an original tube member is now being advanced.
Referring to FIGS. 5A to 6C, an example of a conventional hydroform process will be described. A die assembly 5 shown in FIG. 5A comprises an upper die 2 having a lower surface serving as a molding surface 1, and a lower mold 4 having an upper surface serving as a molding surface 3. An original tube member, such as a steel pipe 6, is placed in the die assembly 5.
After that, as shown in FIG. 5B, a pressurized liquid (e.g. water) is supplied into the steel pipe 6, thereby expanding the pipe 6 by internal pressure. Thus, the steel pipe 6 is expanded in a width direction. The expanded pipe 6 is pressed against the molding surfaces 1 and 3 of the die assembly 5. As a result, the reinforcing member 7 having the closed cross section as shown in FIG. 5C is formed.
The reinforcing member 7, which has a closed section obtained by hydroforming the steel pipe 6, has a peripheral continuous wall. This peripheral wall is hardened by the work hardening effect resulting from circumferential extension of the wall. As a result, the reinforcing member 7 is characterized in that it has a thin wall thickness and a high rigidity.
The shape of the reinforcing member 7 may be complicated by comprising indents and/or protrusions in the section, as shown in FIG. 5C, as required.
To form a reinforcing member 7 with a complicated cross section, it is necessary to use an upper die 2 and lower die 4 having respective complicated molding surfaces corresponding to the cross section of the reinforcing member 7.
However, dies of certain sectional shapes may interrupt the expansion of the steel pipe 6, since only parts of the dies are brought into contact with the outer peripheral surface of the steel pipe 6 during a hydroform process.
For example, the die assembly 5 shown in FIG. 5A and 5B has a molding surface 3 in the form of a trapezoidal recess. The other molding surface 1 has an indent 1a, projection 1b, stepped portion 1c, etc.
During hydroforming, therefore, some parts of the steel pipe 6 are brought into contact with the molding surfaces 1 and 3 before other parts, as shown in FIG. 6A. For example, a first corner portion X1 near the tip of the projection 1b of the inner surface of the die assembly 5, an inner surface X2 opposing the corner portion X1, a second corner portion Y1 adjacent to the corner portion X1, and an inner surface Y2 opposing the corner portion Y1 are brought into contact with the outer periphery of the steel pipe 6 before other portions. In this example, the projection 1b corresponds to “predetermined portion of a die” stated in the present invention.
After that, tube expansion is advanced as shown in FIG. 6B. During the expansion process, in the molding area L1 between the corner portion X1 and inner surface X2 of the projection 1b, the steel pipe 6 is expanded, kept in contact with the corner portion X1 and inner surface X2 and held by these portions. Also in the stepped molding area L2 between the corner portion Y1 and inner surface Y2, the steel pipe 6 is expanded, kept in contact with the corner portion Y1 and inner surface Y2 and held by these portions.
In this conventional case, suppose that hydroforming is started by applying hydraulic pressure to the steel pipe 6, set in the die assembly 5, from inside. While the steel pipe 6 is being expanded by the hydraulic pressure, it is brought into contact with the corner portions X1 and Y1 of the projection 1b, the inner surface X2 at one side, and the inner surface Y2 at the other side. As a result, the outer periphery of the steel pipe 6 is held in the die assembly 5 by friction at the corner portions X1 and Y1 and inner surfaces X2 and Y2, as is shown FIG. 6A.
Since the steel pipe 6 is held at the corners X1 and Y1 of the die assembly 5, it is prevented from uniform extension in the circumference, and hence from uniformly expanding, as is shown in FIGS. 6B and 6C.
Specifically, the to-be-expanded steel pipe 6 is held by friction at the corner portions X1 and Y1 and inner surfaces X2 and Y2 in the molding areas L1 and L2. Accordingly, the entire steel pipe 6 is not uniformly expanded. The extension of the peripheral wall of the steel pipe 6 is advanced between the corner portion X1 and inner surface X2, and the corner portion Y1 and inner surface Y2. In other words, the degree of extension of portions in the molding areas L1 and L2 is greater than that of the other portions.
If the degree of extension of some portions of the steel pipe 6 is greater than that of the other portions during the hydroform process, the wall thickness of the resultant steel pipe 6 is circumferentially non-uniform, as shown in FIG. 6C. In this case, the wall thickness t2 of the resultant steel pipe 6 in the molding areas L1 and L2 is thin, whereas the wall thickness t1 of the portions corresponding to the corner portions X1 and Y1 and inner surfaces X2 and Y2 is thick.
Thus, in a hydroform product having a complicated cross section, the wall thickness is liable to be circumferentially non-uniform, which makes it difficult to obtain a predetermined rigidity. Moreover, if a portion of the steel pipe 6 is extremely extended, it becomes extremely thin and hence may be damaged.
To avoid this problem, it has been proposed that lubricating oil be applied between the die assembly 5 and steel pipe 6, thereby making their contact portions slippery. However, this countermeasure is not sufficient, and a further improvement is demanded.